Known cancer chemotherapy agents target both normal and cancerous tumor cells. This lack of selectivity for tumor cells results in cytotoxicity to the normal cells and is one of the major causes of chemotherapeutic failure in the treatment of cancer. Further, advanced stage and chemotherapeutic agent resistant tumors may be difficult to treat with know chemotherapeutic agents such as for example but not limited to carboplatin or paclitaxel (docitaxel).
Folates are members of the B Class of vitamins that are cofactors for the synthesis of nucleotide precursors. serine and methionine in one-carbon transfer reactions. Since mammals cannot synthesize folates de novo, cellular uptake of these derivatives is essential for cell growth and tissue regeneration. Reflecting their hydrophilic anionic character, folates do not cross biological membranes by diffusion alone. Accordingly, mammalian cells have evolved sophisticated membrane transport systems for facilitating accumulation of folates.
The ubiquitously expressed reduced folate carrier (RFC) is the major transport system for folates in mammalian cells and mediates concentrative uptake of folate substrates. RFC is a member of the major facilitator superfamily of transporters and is an integral transmembrane protein with micromolar affinity for its physiologic substrate, 5-methyl tetrahydrofolate. Importantly, RFC is also the primary transporter of clinically relevant antifolate drugs used for cancer including methotrexate (MTX), raltitrexed (ZD 1694, Tomudex) (RTX), and pemetrexed (LY231514, Alimta) (PMX). Loss of RFC levels or function is a common mode of antifolate resistance. While a previously unrecognized proton-coupled folate transporter (PCFT) was recently reported to contribute to folate absorption in the duodenum, its tissue-specificity and overall role in folate homeostasis are not clear yet.
The family of folate receptors (FRs) represents yet another mode of folate uptake into mammalian cells. The FRs are high affinity folate binding proteins encoded by three distinct genes, designated FR alpha, FR beta and FR gamma, localized to chromosome 11q13.3-q13.5. In contrast to RFC and PCFT, FR alpha and FR beta are anchored in plasma membranes by a glycosyl phosphatidylinositol (GPI) anchor. FR gamma contains no GPI anchor and is secreted. Whereas FR alpha and FR beta (but not FR gamma) mediate cellular accumulation of folate at low (nanomolar) concentrations by receptor-mediated endocytosis, these homologous proteins show differences in binding affinities for reduced folate substrates.
The high affinity FRs offer a potential means of selective tumor targeting, given their restricted pattern of tissue expression and function. For instance, FR alpha is expressed on the apical membrane surface of normal tissues such as kidney, placenta, and choroid plexus, whereas FR beta is expressed in placenta, spleen, and thymus. Importantly, FR alpha is overexpressed in a number of carcinomas including up to 90% of ovarian cancers. Close associations were reported between FR alpha expression levels with grade and differentiation status of ovarian tumors. FR alpha in normal tissues (unlike tumors) is reported to be inaccessible to the circulation. FR beta is expressed in a wide range of myeloid leukemia cells. FR beta in normal hematopoetic cells differs from that in leukemia cells in its inability to bind folate ligand.
Folate-conjugated cytotoxins, liposomes, or radionuclides, or cytoxic antifolates have all been used to target FRs. Unfortunately, for most folate-based therapeutics such as classical antifolates (including RTX, PMX, and lometrexol (LMX)), tumor selectivity is lost since substrates are shared between FRs and the ubiquitously expressed RFC. Indeed, this likely explains the severe myelosupression encountered in phase 1 studies with LMX.
If, a FR-targeted ligand were itself cytotoxic without RFC activity, selective tumor targeting would ensue. Antifolates that selectively target FRs over RFC have been described including CB3717 and, more recently, cyclopenta[g]quinazoline antifolates BGC638 and BGC945, all of which potently inhibit thymidylate synthase (TS) within cells. When BGC945 was tested in mice, there was no toxicity to normal tissues, as reflected in weight loss, nor were there any macroscopic signs of toxicity to major organs, consistent with the premise that FR targeting is highly selective.
As is known by those skilled in the art, FRs such as FR alpha and FR beta are overexpressed on a substantial amount of certain surfaces of a number of types of cancerous tumors. FR alpha is known to be overexpressed in ovarian, endometrial, kidney, lung, mesothelioma, breast and brain tumors. FR beta is known to be overexpressed in acute myeloid leukemias
In most normal cells, the FRs are not present. In most normal cells, folic acid is not taken up by way of a reduced folate carrier (RFC) system. Uptake of folates and antifolates by tissues and tumors is primarily by the ubiquitously expressed RFC system. In light of the specificity of folic acid, conjugates of folic acid have been used to selectively deliver toxins, liposomes, imaging agents, and cytotoxic agents to FR expressing tumors. The major limitation of the folic acid conjugates is that they require cleavage from the folic acid moiety to release, for example, the cytotoxic agent. Cleavage of the cytotoxic agent moiety from the folic acid conjugate is often difficult to achieve and the anti-tumor activity is hindered or is nonexistent as a result of the inability or reduced ability to release the cytotoxic agent. Another limitation of the folic acid conjugates entails premature release of the cytotoxic agent during transport and before reaching the cancerous tumor. The premature release thus leads to undesired toxicity in normal cells.
The FRs alpha and beta represent another mode of folate uptake and are considered by those skilled in the art to be potential chemotherapeutic targets for selective tumor uptake. US Patent Application Publication No. US 2008/0045710 A1, published Feb. 21, 2008 (Aleem Gangjee) describes compounds for treating cancer tumors wherein fused cyclic pyrimidines are used to selectively target FRs of cancerous tumors that express FR alpha and FR beta and that inhibit glycinamide ribonucleotide formyltransferase (GARFTase) enzyme. The compounds are not significantly taken up by a cell or tissue using the RFC system.
There is a need for single compounds having potent anti-tumor activity that selectively target FR alpha and FR beta of cancerous cells, that inhibit GARFTase in cancerous cells, and that have a negligible substrate activity for RFC.